There is a known technique to obtain optical disks for DVD by bonding a plurality of substrates having recording layers to each other thereby to achieve multilayer recording
The Applicant has proposed a method and an apparatus for bonding of optical disks in a technique described in the International Publication No. WO 97/35720.
In the disclosed technique, a pair of substrates are held to confront each other via a relatively narrow interval. An adhesive discharge nozzle is inserted into the interval of the substrates. While the adhesive is injected out through the discharge nozzle, the pair of substrates are rotated in a planar direction (rotated around a center axis of the substrates) to thereby arrange the adhesive annularly in the interval. The interval of the substrates is reduced then and at the same time, the substrates are further rotated. In consequence, the annularly injected adhesive is spread in a radial direction to thereby fill the interval with a thin adhesive layer. The pair of substrates is bonded via the adhesive layer when the adhesive layer is set through the projection of ultraviolet rays. An optical disk is obtained in this manner.
According to the above-discussed method, the adhesive arranged annularly in the interval of the substrates is expanded in the radial direction while being kept in contact with the substrates, so that the thin adhesive layer of a correct thickness can be formed efficiently. Moreover, the method is advantageous in that air bubbles or gaps are difficult to bring about between the adhesive layer and the substrate, excessive use of the adhesive is eliminated, etc.
However, the above method has disadvantages as well. Specifically, the method needs the arrangement of the substrates in an apparatus that can apply or feed the adhesive while the pair of substrates is held with respective bond faces opposed to each other. It is additionally required in the method to send a pair of substrates holding the adhesive layer therebetween to a setting apparatus in order to set the adhesive and to handle the substrates efficiently and correctly to prevent bad influences upon the processing accuracy of the optical disks.
For instance, since substrates of each pair are manufactured or prepared separately due to a difference in material and structure of recording layers, it is troublesome to correctly combine the plurality of kinds of substrates and feed them to a predetermined position. Even after the adhesive layer is formed at the interval of the substrates, there is a possibility that the substrates and adhesive layer will be moved or deformed before the adhesive layer is set. The substrates and adhesive layer should be treated to avoid such inconveniences, which is a difficult task.